1. Field of the Invention
The invention relates to wavelength-division multiplex optical transmission systems, and more particularly to a wavelength-division multiplex optical transmission system for transmitting signals to be supplied individually to a plurality of optical receiving parts and signals to be supplied equally thereto on a wavelength-division-multiplexed optical signal.
2. Description of the Background Art
Recently, in fields such as optical CATV systems, data communications services providing digital signals for data communications are offered together with conventional video-broadcast services. In such services, it is specifically required to offer a specific data communications service to each of a plurality of optical receiving parts while offering the same video-broadcast services equally to all optical receiving parts. Further, due to increasing data amount in data communications, a system for supplying large communications data individually to each optical receiving part at the same time of offering the conventional video-broadcast service is required and is under study.
A wavelength-division multiplex transmission technique is appropriate for simultaneously supplying large communications data to each of the optical receiving parts. With the technique, a plurality of signals are each superimposed on optical signals having different wavelengths from each other and multiplexed to become a single optical signal. After being transmitted through an optical transmission path such as an optical fiber, the multiplexed optical signal is separated for each wavelength, and thereby the optical signals before multiplexing are obtained.
Conventionally, a wavelength-division multiplex optical transmission system using the above technique is disclosed in xe2x80x9cOPTCOMxe2x80x9d, No. 115, June 1999 issue, pp. 46 to 50. FIG. 7 is a block diagram showing the structure of the conventional wavelength-division multiplex optical transmission system. The system is described next below.
In FIG. 7, the conventional wavelength-division multiplex optical transmission system includes: a data communications central station 109; a video-broadcast central station 209; a repeater station 309; a plurality of optical receiving parts 411 to 41n; and a plurality of optical fibers 570, 580, and 591 to 59n. The data communications central station 109 is provided with a plurality of optical modulators 111 to 11n and a wavelength-division-multiplexing part 120. The video-broadcast central station 209 is provided with an optical modulator 290, and the repeater station 309 is provided with a demultiplexing part 310, a branching part 380, and a plurality of multiplexing parts 391 to 39n. The operation of the system is described next below.
In the data communications central station 109, a plurality of communications signals 11 to 1n, which are to be supplied individually to each of the plurality of optical receiving parts 411 to 41n as the data communications services, are inputted to the plurality of optical modulators 111 to 11n, respectively. The plurality of optical modulators 111 to 11n each output optical signals modulated by the incoming communications signals 11 to 1n and each having different wavelengths xcex1, xcex2, . . . , xcexn. The output optical signals are multiplexed in the wavelength-division-multiplexing part 120 and transmitted to the repeater station 309 through the optical fiber 570. In the video-broadcast central station 209, a broadcast signal 20, which is to be supplied equally to the plurality of optical receiving parts 411 to 41n as the video-broadcast service, is inputted to the optical modulator 290. The optical modulator 290 outputs an optical signal modulated by the incoming broadcast signal 20 and having a wavelength xcexb that is different from any of the wavelengths of the optical signals outputted from the optical modulators 111 to 11n. The output optical signal is transmitted to the repeater station 309 through the optical fiber 580. In the repeater station 309, the optical signal transmitted through the optical fiber 570 from the data communications central station 109 is separated, based on the wavelengths, in the demultiplexing part 310 into the optical signals having the wavelengths xcex1, xcex2, . . . , xcexn. Also, the optical signal transmitted through the optical fiber 580 from the video-broadcast central station 209 is branched in the branching part 380 to a plurality of optical signals all having the wavelength xcexb. Each of the separated optical signals separated in the demultiplexing part 310 is supplied to the respective multiplexing parts 391 to 39n, and therein, multiplexed with each of the branched optical signals also supplied thereto. The multiplexed optical signals are each transmitted through the optical fibers 591 to 59n to the optical receiving part 411 to 41n. That is to say, the optical signal obtained by multiplexing the optical signal having the xcex1 wavelength and the optical signal having the xcexb wavelength is supplied to the optical receiving part 411, and the optical signal obtained by multiplexing the optical signal having the xcex2 wavelength and the optical signal having the xcexb wavelength is supplied to the optical receiving part 412. Other optical receiving parts 413 to 41n are similarly supplied with the multiplexed optical signals. Each of the optical receiving parts 411 to 41n separates, by using a wave separator not shown in the drawing, the optical signal supplied thereto into the optical signal having any one of the wavelengths xcex1, xcex2, . . . , xcexn that carries the corresponding communications signal 11 to 1n, and the optical signal having the xcexb wavelength that carries the broadcast signal 20. Thereafter, each of the optical receiving parts 411 to 41n converts, by using two optical receivers not shown in the drawing, each of the separated optical signals into electrical signals. Note, although omitted in the drawing, optical amplification is carried out in each part of the system, as required, to compensate for the transmission loss and the splitting loss.
As described, according to the conventional wavelength-division multiplex optical transmission system, path selection is made based on the wavelength, thereby enabling simultaneous transmission of the communications signals individually supplied to each of the optical receiving parts and the broadcast signal equally supplied to the optical receiving parts. In other words, with such system, the video-broadcast service offered to all optical receiving parts and the data communications service offered to a specific optical receiving part can be combined.
In the system shown in FIG. 7, however, to combine the video-broadcast service offered to all optical receiving parts and the data communications service offered to a specific optical receiving part, the repeater station 309 is required to first separate, for each wavelength, the received optical signal obtained by wavelength-division-multiplexing the optical signals carrying the communications signals, and then again multiplexing each separated signal with the optical signal carrying the broadcast signal 20. Accordingly, the problem comes up that the structure of the repeater station 309 becomes complex. Moreover, each of the optical receiving parts 411 to 41n is required to be provided with the wave separator and two optical receivers in order to receive the optical signal obtained by multiplexing two types of optical signal, i.e., the optical signal for carrying the respective communications signals 11 to 1n; and the optical signal for carrying the broadcast signal 20. As a result, another problem comes up that the cost of the optical receiving parts 411 to 41n increases.
To solve the problems above, considered is a method that each of the communications signals 11 to 1n is previously frequency-division-multiplexed with the broadcast signal 20 to become an electrical signal, and the frequency-division-multiplexed electrical signals are each superimposed on optical signals having predetermined wavelengths for wavelength-division multiplex optical transmission. According to the method, an optical signal of a different wavelength for transmitting the broadcast signal 20 is not required. Indeed, the broadcast signals 20 are transmitted on the optical signals each carrying the communications signals 11 to 1n. Accordingly, an optical transmission path to the repeater station can be implemented by a single optical fiber, and the repeater station by only the branching part 310. Further, each of the optical receiving parts 411 to 41n can reproduce, only by converting the received optical signal into an electrical signal, the electrical signal in which one of the communication signals 11 to 1n and the broadcast signal 20 are frequency-division-multiplexed. Therefore, each of the optical receiving parts 411 to 41n does not require a costly wave separator but requires only one optical receiver. In such manner, the total structure of the system can be simplified and the cost of the optical receiving parts 411 to 41n can be reduced.
With the above described system, however, each of the communication signals 11 to 1n and the broadcast signal 20 are required to be frequency-division-multiplexed in advance. Generally, there are many cases that the data communications service and the video-broadcast service are offered by different providers (or broadcast media). That is to say, there are many cases that the sources of the communications signals 11 to 1n and the broadcast signal 20 are far in distance each other. In such cases, the communications signals 11 to 1n or the broadcast signal 20 is required to be transmitted from its source as an electrical signal to a predetermined frequency-division-multiplexing part located apart from the source. Since a transmission attenuation of the electrical signal is larger than that of the optical signal, the sources of the communications signals 11 to 1n and the broadcast signal 20 should be kept not so far apart, disadvantageously resulting in restrictions, in distance, on placement of the sources.
Therefore, an object of the present invention is to provide a wavelength-division multiplex optical transmission system with simple structure and low cost, capable of simultaneously offering a broadcast service to all optical receiving parts and a communications service to a specific optical receiving part, and having flexibility in placement of each signal source of communications signal and broadcast signal.
The present invention has the following features to achieve the object above.
A first aspect of the present invention is directed to a wavelength-division multiplex optical transmission system for simultaneously transmitting first signals to be supplied individually to a plurality of optical receiving parts and a second signal to be supplied equally thereto on a wavelength-division-multiplexed optical signal. A system in accordance with the first aspect of the present invention comprises: a plurality of preliminary optical modulators, to which the plurality of first signals are each inputted, each outputting optical signals having different wavelengths from each other and being modulated by the plurality of first signals; a wavelength-division-multiplexing part multiplexing the optical signals outputted by the plurality of preliminary optical modulators; a subsequent optical modulator, to which the second signal is inputted, modulating the optical signal received from the wavelength-division-multiplexing part by the second signal so as to collectively modulate the optical signals being multiplexed; a demultiplexing part separating the optical signal modulated by the subsequent optical modulator for each wavelength; and the plurality of optical receiving parts each provided for receiving each of the optical signals separated and outputted by the demultiplexing part, and converting the received optical signal into an electrical signal.
As described above, in the first aspect, the second signal can be transmitted on the same optical signal that transmits the first signals without using the optical signal having a wavelength unique thereto. Further, as the second signal and the first signals are each inputted to the different modulators, the electrical signals can be each inputted to the modulators without attenuation even if the signal sources of the electrical signals are located away from each other. As a result, the total system structure can be simplified, the costs can be reduced, and further the flexible placement of the signal sources can be achieved. Note that the first signals are signals each individually supplied to the plurality of optical receiving parts such as communications signals and the second signal is the signal equally supplied to the plurality of optical receiving parts such as a broadcast signal.
According to a second aspect, in the first aspect, a frequency band for the second signal does not overlap with any of frequency bands for the plurality of first signals.
As described above, in the second aspect, the electrical signals obtained in the plurality of optical receiving parts each include the first signal and the second signal in different frequency bands. Accordingly the first signal and the second signal can be separated each other and extracted by using a band-pass filter.
According to a third aspect, in the first aspect, the wavelength-division-multiplexing part and the demultiplexing part are connected to each other through an optical transmission path, and the subsequent optical modulator is provided at an arbitrary place on the optical transmission path.
As described above, in the third aspect, the subsequent optical modulator to which the second signal is inputted can be provided at an arbitrary place regardless of the distance between the plurality of preliminary optical modulators to which the first signals are each inputted. Accordingly, the signal sources of the first signals and the second signal can be placed at arbitrary places regardless of each other""s distance.
According to a fourth aspect, in the first aspect, the system further comprises: a plurality of first spread spectrum modulators each spread-spectrum-modulating each of the plurality of first signals with spreading codes for output to the plurality of preliminary optical modulators; a second spread spectrum modulator spread-spectrum-modulating the second signal with a spreading code for output to the subsequent optical modulator; and a plurality of despreading parts each despreading each of the electrical signals converted in the plurality of optical receiving parts, wherein the spreading code used by the second spread spectrum modulator is different from any of the spreading codes used by the first spread spectrum modulators.
As described above, in the fourth aspect, the first signals and the second signal are each spread-spectrum-modulated by different spreading codes for transmission. It is thus possible to separately extract the first signal and the second signal by despreading each of the signals being carried on the optical signal by using respective spreading codes. Accordingly, the frequency bands for the first signals and the second signal can be set at will without restrictions, thereby enabling efficient utilization of the frequency bands.
According to a fifth aspect, in the first aspect, the system further comprises: a subsequent wavelength-division-multiplexing part provided between the subsequent optical modulator and the demultiplexing part, and multiplexing the optical signal outputted by the subsequent optical modulator and other one or more optical signals, wherein wavelengths of the other one or more optical signals being multiplexed differ from any of the wavelengths of the optical signals outputted by the plurality of preliminary optical modulators, and differ from each other.
As described above, in the fifth aspect, optical signals to be supplied to the optical receiving parts which are in no need of the second signal can be transmitted together with the optical signal carrying the second signal by sharing a transmission path and a repeater station. Accordingly, the system is efficiently used, resulting in simplification of the system.
A sixth aspect of the present invention is directed to a wavelength-division multiplex optical transmission system for simultaneously transmitting a first signal to be supplied individually to any of a plurality of optical receiving parts and a second signal to be supplied equally thereto on a wavelength-division-multiplexed optical signal. A system in accordance with the sixth aspect of the present invention comprises: a switch, to which the first signal is inputted, selecting a destination of the first signal from among said plurality of optical receiving parts; a plurality of preliminary optical modulators each provided in correspondence with each of the optical receiving parts, each being supplied with said first signal from said switch based on the result of selection made thereby, and each outputting optical signals having different wavelengths from each other and being modulated by the first signal when being supplied therewith and without modulation when not being supplied therewith; a wavelength-division-multiplexing part multiplexing the optical signals outputted by the plurality of preliminary optical modulators; a subsequent optical modulator, to which the second signal is inputted, modulating the optical signal received from the wavelength-division-multiplexing part by the second signal so as to collectively modulate the optical signals being multiplexed; a demultiplexing part separating the optical signal modulated by the subsequent optical modulator for each wavelength; and the plurality of optical receiving parts each provided for receiving each of the optical signals separated and outputted by the demultiplexing part, and converting the received optical signal into an electrical signals, wherein a frequency band for the second signal does not overlap with a frequency band for the first signal.
As described above, in the sixth aspect, a path for the first signal is easily selected by the switch. Accordingly, the first signal can be selectively supplied only to a desired optical receiving part while equally supplying the second signal to all optical receiving parts.
According to a seventh aspect, in the sixth aspect, the wavelength-division-multiplexing part and the demultiplexing part are connected to each other through an optical transmission path, and the subsequent optical modulator is provided at an arbitrary place on the optical transmission path.
As described above, in the seventh aspect, the electrical signal obtained in the optical receiving part that is to be supplied with both of the first and second signals include the first and second signals in different frequency bands. Accordingly the first signal and the second signal can be separated each other and extracted by using a band-pass filter.
An eighth aspect of the present invention is directed to a wavelength-division multiplex optical transmission device for simultaneously transmitting first signals to be supplied individually to a plurality of optical receiving parts and a second signal to be supplied equally thereto on a wavelength-division-multiplexed optical signal. A device in accordance with the eighth aspect of the present invention comprises: a plurality of preliminary optical modulators, to which the plurality of first signals are each inputted, each outputting optical signals having different wavelengths from each other and being modulated by the plurality of first signals; a wavelength-division-multiplexing part multiplexing the optical signals outputted by the plurality of preliminary optical modulators; and a subsequent optical modulator, to which the second signal is inputted, modulating the optical signal received from the wavelength-division-multiplexing part by the second signal so as to collectively modulate the optical signals being multiplexed.
As described above, in the eighth aspect, the second signal can be transmitted on the same optical signal that transmits the first signals without using the optical signal having a wavelength unique thereto. Further, as the second signal and the first signals are each inputted to the different modulators, the electrical signals can be each inputted to the modulators without attenuation even if the signal sources of the electrical signals are located away from each other. As a result, the structure of the device can be simplified, the costs can be reduced, and further the flexible placement of the signal sources can be achieved.
According to a ninth aspect, in the eighth aspect, a frequency band for the second signal does not overlap with any of frequency bands for the plurality of first signals.
As described above, in the ninth aspect, the electrical signals obtained in the plurality of optical receiving parts each include the first signal and the second signal in different frequency bands. That is to say, the device can transmit the optical signal from which the first signal and the second signal can be separated each other and extracted by using a band-pass filter.
According to a tenth aspect, in the eighth aspect, the device further comprises: a plurality of first spread spectrum modulators each spread-spectrum-modulating each of the plurality of first signals with spreading codes for output to the plurality of preliminary optical modulators; and a second spread spectrum modulator spread-spectrum-modulating the second signal with a spreading code for output to the subsequent optical modulator, wherein the spreading code used by the second spread spectrum modulator is different from any of the spreading codes used by the first spread spectrum modulators.
As described above, in the tenth aspect, the first signals and the second signal are each spread-spectrum-modulated by different spreading codes for transmission. It is thus not required to set different frequency bands for both signals for transmission. Accordingly, the frequency bands for the first signals and the second signal can be set at will without restrictions, thereby enabling efficient utilization of the frequency bands.
An eleventh aspect of the present invention is directed to a wavelength-division multiplex optical transmission device for simultaneously transmitting a first signal to be supplied individually to any of a plurality of optical receiving parts and a second signal to be supplied equally thereto on a wavelength-division-multiplexed optical signal. A device in accordance with the eleventh aspect of the present invention comprises: a switch, to which the first signal is inputted, selecting a destination of the first signal from among the plurality of optical receiving parts; a plurality of preliminary optical modulators each provided in correspondence with each of the optical receiving parts, and each outputting optical signals having different wavelengths from each other and being modulated by the first signal when being supplied therewith and without modulation when not being supplied therewith; a wavelength-division-multiplexing part multiplexing the optical signals outputted by the plurality of preliminary optical modulators; and a subsequent optical modulator, to which the second signal is inputted, modulating the optical signal received from the wavelength-division-multiplexing part by the second signal so as to collectively modulate the optical signals being multiplexed, wherein a frequency band for the second signal does not overlap with a frequency band for the first signal.
As described above, in the eleventh aspect, a path for the first signal is easily selected by the switch. Accordingly, the first signal can be selectively supplied only to a desired optical receiving part while equally supplying the second signal to all optical receiving parts.
A twelfth aspect of the present invention is directed to a wavelength-division multiplex optical transmission method for simultaneously transmitting first signals to be supplied individually to a plurality of optical receiving parts and a second signal to be supplied equally thereto on a wavelength-division-multiplexed optical signal. A method according to the twelfth aspect of the present invention comprises; outputting optical signals having different wavelengths from each other and being modulated by the plurality of first signals; multiplexing the optical signals outputted in the outputting; and modulating the optical signal obtained in the multiplexing by the second signal so as to collectively modulate the optical signals being multiplexed.
As described above, in the twelfth aspect, the second signal can be transmitted on the same optical signal that transmits the first signals without using the optical signal having a wavelength unique thereto. Further, as the second signal and the first signals are each superimposed on the optical signals in the different steps, the electrical signals can be each superimposed on the optical signals without attenuation even if the signal sources of the electrical signals are located away from each other. As a result, the structure of the device can be simplified, the costs can be reduced, and further the flexible placement of the signal sources can be achieved.
According to a thirteenth aspect, in the twelfth aspect, a frequency band for the second signal does not overlap with any of frequency bands for the plurality of first signals.
As described above, in the thirteenth aspect, the electrical signals obtained in the plurality of optical receiving parts each include the first signal and the second signal in different frequency bands. That is to say, the device can transmit the optical signal from which the first signal and the second signal can be separated each other and extracted by using a band-pass filter.
According to a fourteenth aspect, in the twelfth aspect, the method further comprises; spread-spectrum-modulating each of the plurality of first signals with spreading codes before the outputting; and spread-spectrum-modulating the second signal with a spreading code before the modulating, wherein the spreading code used in the second signal spread spectrum modulating is different from any of the spreading codes used in the first signal spread spectrum modulating.
As described above, in the fourteenth aspect, the first signals and the second signal are each spread-spectrum-modulated by different spreading codes for transmission. It is thus not required to set different frequency bands for both signals for transmission. Accordingly, the frequency bands for the first signals and the second signal can be set at will without restrictions, thereby enabling efficient utilization of the frequency bands.
A fifteenth aspect of the present invention is directed to a wavelength-division multiplex optical transmission method for simultaneously transmitting a first signal to be supplied individually to any of a plurality of optical receiving parts and a second signal to be supplied equally thereto on a wavelength-division-multiplexed optical signal. A method according to the twelfth aspect of the present invention comprises: selecting a destination of the first signal from among the plurality of optical receiving parts: outputting optical signals having different wavelengths from each other and being modulated by the first signal for the optical receiving part selected as the destination and without modulation for other optical receiving parts; multiplexing the optical signals outputted in the outputting; and modulating the optical signal obtained in the multiplexing by the second signal so as to collectively modulate the optical signals being multiplexed, wherein a frequency band for the second signal does not overlap with a frequency band for the first signal.
As described above, in the fifteenth aspect, a path for the first signal is easily selected. Accordingly, the first signal can be selectively supplied only to a desired optical receiving part while equally supplying the second signal to all optical receiving parts.
A sixteenth aspect of the present invention is directed to a wavelength-division multiplex optical transmission device for simultaneously transmitting a plurality of first signals to be supplied individually to a plurality of optical receiving parts and a second signal to be supplied equally thereto on an optical signal. A device in accordance with the sixteenth aspect of the present invention comprises: input means for inputting an optical signal obtained by wavelength-division-multiplexing a plurality of optical signals having different wavelengths from each other and each being intensity-modulated by each of the plurality of first signals; input means for inputting the second signal; and intensity-modulating means for intensity-modulating the wavelength-division-multiplexed optical signal so as to collectively modulate the plurality of optical signals being multiplexed, by the second signal.
As described above, in the sixteenth aspect, the second signal can be transmitted on the same optical signal that transmits the first signals without using the optical signal having a wavelength unique thereto. Further, as the second signal and the first signals are each inputted to the different modulators, the electrical signals can be each inputted to the modulators without attenuation even if the signal sources of the electrical signals are located away from each other. As a result, the structure of the device can be simplified, the costs can be reduced, and further the flexible placement of the signal sources can be achieved.
According to a seventeenth aspect, in the sixteenth aspect, a frequency band for the second signal does not overlap with any of frequency bands for the plurality of first signals.
As described above, in the seventeenth aspect, the electrical signals obtained in the plurality of optical receiving parts each include the first signal and the second signal in different frequency bands. That is to say, the device can transmit the optical signal from which the first signal and the second signal can be separated each other and extracted by using a band-pass filter.
An eighteenth aspect of the present invention is directed to a wavelength-division multiplex optical transmission device for simultaneously transmitting a plurality of first signals to be supplied individually to a plurality of optical receiving parts and a second signal to be supplied equally thereto on an optical signal. A device in accordance with the eighteenth aspect of the present invention comprises: input means for inputting an optical signal obtained by wavelength-division-multiplexing a plurality of optical signals having different wavelengths from each other and each being intensity-modulated by each of the plurality of first signals which is spread-spectrum-modulated with spreading codes; spread spectrum modulating means for spread-spectrum-modulating the second signal with a spreading code; and intensity-modulating means for intensity-modulating the wavelength-division-multiplexed optical signal by the second signal so as to collectively modulate the plurality of optical signals being multiplexed, wherein the spreading code used for spread-spectrum-modulating the second signal is different from any of the spreading codes each used for spread-spectrum-modulating the plurality of first signals.
As described above, in the eighteenth aspect, the first signals and the second signal are each spread-spectrum-modulated by different spreading codes for transmission. It is thus not required to set different frequency bands for both signals for transmission. Accordingly, the frequency bands for the first signals and the second signal can be set at will without restrictions, thereby enabling efficient utilization of the frequency bands.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.